1. Field of the Invention
The present invention relates generally to tunable lasers and other tunable optical signal sources and more particularly to a method and system for compensating for a temperature induced shift in an etalon's Fabry-Perot output characteristics relative to a channel grid on which the laser wavelength lock is to be maintained.
2. Description of the Related Art
Optical fiber communications systems provide for low loss and very high information carrying capacity. In practice, the bandwidth of optical fiber may be utilized by transmitting many distinct channels simultaneously using different carrier wavelengths. The associated technology is called wavelength division multiplexing (WDM). In a narrow band WDM system, eight, sixteen or more different wavelengths are closely spaced to increase fiber transmission capacity.
The wavelength bandwidth that any individual channel occupies depends on a number of factors, including the impressed information bandwidth, and margins to accommodate carrier frequency drift, carrier frequency uncertainty, and to reduce possible inter-channel cross-talk due to non-ideal filters.
To maximize the number of channels, lasers with stable and precise wavelength control are required to provide narrowly spaced, multiple wavelengths. However, in practice, a laser generates light over rather broad bandwidths referred to as the laser gain curve. The only longitudinal-mode discrimination in conventional Fabry-Perot lasers is provided by the gain spectrum itself. Since the laser cavity is a type of Fabry-Perot interferometer, the energy output over the gain curve is not continuous but occurs at discrete, closely spaced frequencies. The output frequencies are based upon the number of discrete longitudinal modes that are supported by the laser cavity. The longitudinal modes will occur at wavelengths such that an integral number of half wavelengths equals the distance between the mirrors of the resonator in the laser. Laser oscillation can occur only at frequencies where the laser gain curve exceeds the losses in the optical path of the resonator. In practice, the broadened laser gain curve exceeds the cavity losses over a large frequency range, on the order of 8 to 10 GHz. As noted above, there will be a number of discrete, closely spaced modes oscillating within this range.
Various approaches have been used to limit the oscillation of a laser to one of the competing longitudinal modes. One approach is a distributed feedback mechanism. Distributed-feedback (DFB) lasers are the most common type of communications laser. A grating integral to the laser structure limits the output to a single frequency. Another of the most common methods requires the use of a frequency selective external cavity/etalon in combination with such a laser device to detect the output wavelength at which the laser is operating and adjust the laser accordingly by varying the temperature of the laser, known as temperature tuning. Such a method allows for wavelength locking of a laser even in the event of changing ambient temperature conditions. The external cavity/etalon laser is most commonly applied to gas tube lasers but has also been applied for very narrow line width lasers such as those needed for laser gyro use. There are a number of types of etalons. In its simplest form, an etalon consists of a quartz glass plate with parallel surfaces that is placed in the laser resonator at a non-normal angle. Internal reflections give rise to interference effects which cause the etalon to behave as a frequency selective transmission filter, passing with minimum loss frequencies close to a transmission peak and rejecting by destructive interference other frequencies. In practice, the transmission peak of the etalon is set to coincide with a particular longitudinal mode resulting in single frequency operation of the laser.
There are problems, however, with wavelength stability of a temperature tuned laser for a long duration due to thermal problems. Both the DFB laser and external cavity/etalon lasers need to make the wavelength insensitive to temperature change. Typically, a Thermo-Electric Cooler (TEC) and thermistor inside the laser package is sufficient to stabilize wavelength. However, with current dense wavelength division multiplexing (DWDM), wavelength stabilization to parts per million control may not be possible utilizing conventional methods. For example, a wavelength shift as illustrated in FIG. 1 can occur. As previously noted, temperature tuning of a laser's wavelength is accomplished by varying the laser chip temperature via the control current supplied to the Thermo-Electric Cooler (TEC) that the laser chip is mounted on within the laser package. As shown in FIG. 1, a laser package will tune the laser chip to maintain the desired maximum output power at the desired wavelength (λ desired), illustrated by point A. As the laser temperature is changed to maintain this point, it is unavoidable due to the proximity of the etalon to the TEC within the laser package that the temperature of the etalon will also change. As the etalon temperature changes, the index of refraction of the material varies and more strongly, the etalon expands or contracts, changing the effective path lengths within the material and thereby changing the interference effects. As the effective path lengths vary, the response curve of the etalon will also vary. The control system of the laser chip will temperature tune the laser chip based on the varied etalon response curve, causing the wavelength output to vary to a different wavelength (λ actual), illustrated by point B. This wavelength shift due to control system error and hysteresis can cause problems in the end system. Since the laser is initially set to deliver, to the end system in which the laser is mounted, a specified power and wavelength output, when the wavelength of the output varies it can cause disruption to the operation of the end-product system and possibly even damage the functionality of the end-product system.
Thus, there exists a need for a method and apparatus for stabilizing and locking on an absolute wavelength of laser light by compensating for a temperature change of the etalon in a temperature tuned laser.